There were widespread reports across the Internet from users saying they could not gain access to their online accounts as a result of the issues. Several attempts to access the system by FOX Business yielded an error message saying the site is “temporarily unavailable,” and pointing customers to ATM machines and bank branches.

The second-biggest U.S. bank by assets issued a tweet to customers on its verified account earlier in the day saying it is “working to resolve” the problems as quickly as possible.

It wasn’t clear what caused the problems and a BofA spokesperson did not immediately respond to requests for comment via telephone and email.

In recent months, several high-profile banks, including BofA, have been the subject of so-called distributed denial-of-service attacks that have snarled access to their websites. As recently as December, the Office of the Comptroller of the Currency has warned banks to have a “heightened sense of awareness” about such attacks.